In general, polyimide resins are heat resistant, but insoluble in solvents except some high-boiling organic solvents and they cannot be directly applied to electronic parts and other substrates to form coatings thereon. It is thus a common practice to form a polyimide resin coating by dissolving a polyamic acid which is a polyimide precursor in an organic solvent to form a coating solution, applying the solution to a substrate as a thin film coating, and heating the coating at relatively high temperatures for an extended period of time for curing. More particularly, a polyimide resin coating is prepared by effecting addition reaction between a tetracarboxylic acid dianhydride and an aromatic diamine in an organic polar solvent, thereby forming a polyamic acid which is a polyimide precursor in solution form, applying it to a substrate, typically in the form of an electronic part as a thin film coating, and heating the coating at high temperatures of at least 300.degree. C. for an extended period of time, thereby effecting dehydration and imidization.
This process includes the high-temperature, long-term heating step which is disadvantageous as a working step, especially from the standpoint of energy consumption. If heating is insufficient, then some polyamic acid would be left in the resulting resin structure, causing the polyimide resin to lose humidity resistance and corrosion resistance. Particularly for insulating protective coatings on electronic parts, such losses of resin attributes would undesirably deteriorate electronic parts to reduce their service life.
One approach for overcoming the above-mentioned problem is to prepare polyimide resins soluble in organic solvents. Then the resin solutions are applied to substrates and heated until the solvents evaporate off, obtaining resin films.
Several methods are known in the art for the preparation of organic solvent-soluble polyimide resins. One method is by heating tetracarboxylic acid dianhydrides and aromatic diamines in such solvents as phenol and halophenol for reaction to thereby form polyimide resins soluble in phenolic solvents (e.g., Japanese Patent Publication Nos. 26878/1972, 65227/1980, 187430/1983, 35026/1985, and 197731/1985). Another method is by reacting specific tetracarboxylic acid dianhydrides and specific diamines to form polyimides soluble in polar solvents having high boiling points such as N-methyl-2-pyrrolidone (e.g., Japanese Patent Publication Nos. 30319/1977, 83228/1986, and 18426/1987).
The polyimide resins prepared by the former method require careful handling and are less safe or hygienic in that when they are used as solutions in phenolic solvents to form coatings, evaporation of the solvent gives off a foul smell like cresol odor and the solvent causes chemical scald to the skin on accidental contact therewith. In turn, the polyimide resins prepared by the latter method have the problem that when they are applied to substrates as solutions in N-methyl-2-pyrrolidone which is highly hygroscopic, the coatings become white turbid due to moisture absorption, resulting in a loss of film strength. In addition, no improvement in working conditions is expected because N-methyl-2-pyrrolidone has a high boiling point so that high-temperature, long-term heating is necessary to completely remove the solvent. Thus, these resins are against the purpose of forming polyimide films of quality by low-temperature, short-term heating.
Therefore, an object of the present invention is to provide a novel and improved polyimide resin solution composition comprising a polyimide resin which is dissolvable in a low-boiling, volatile organic solvent to form a solution which is ready to form a polyimide resin film having improved adherence, heat resistance, electrical and mechanical properties through a low-temperature, short-term heating step. Another object of the invention is to provide such a polyimide resin solution composition which is shelf stable and safe.